The duration with which a wireless terminal (e.g., a cellular telephone, a hand-held data entry device, a cellular modem, a cordless telephone, a hand-held Global Positioning System receiver, etc.) can operate is limited by the product of two factors: (1) the power requirement of the wireless terminal, and (2) the capacity of the energy storage device that powers the wireless terminal.
Numerous technologies have been invented to reduce the power consumption of wireless terminals. These include, for example, the development of low-power CMOS devices and the development of time-division multiplexed/time-division multiple access ("TDM/TDMA") and time-division duplex ("TDD") protocols. TDM/TDMA and TDD protocols reduce power consumption by enabling a wireless terminal's radio transmitter to enter a low-power idle mode when the transmitter is not transmitting and the wireless terminal's radio receiver to enter a low-power idle mode when the receiver is not receiving.
For example, FIG. 1 depicts a block diagram of an illustrative wireless terminal that is typical of a time-division wireless terminal in the prior art. Wireless terminal 100 comprises: an energy storage device such as battery 101, time-division receiver 103 and time-division transmitter 105. Battery 101 is typically an electro-chemical energy storage device that powers receiver 103 and transmitter 105, in well-known fashion.
Time-division receiver 103 is compliant with a TDM/TDMA or TDD protocol, such as the IEEE 802.11 Wireless LAN protocol, in well-known fashion. The salient characteristic of receiver 103 is that it is always in either of one or two modes: receive mode or idle mode. When receiver 103 is in receive mode, it is capable of receiving RF signals from a remote transmitter and draws approximately 250 milliwatts from battery 101. When receiver 103 is in idle mode, it is not capable of receiving RF signals but only draws 5 milliwatts from battery 101. As is well-known in the prior art, the particular protocol to which wireless terminal 100 operates determines how and when receiver 103 toggles between receive mode and idle mode.
Time-division transmitter 105 is compliant with a TDM/TDMA or TDD protocol, such as the IEEE 802.11 Wireless LAN protocol, in well-known fashion. The salient characteristic of transmitter 105 is that it is always in either of one of two modes: transmit mode or idle mode. When transmitter 105 is in transmit mode, it is capable of transmitting RF signals to a remote receiver and draws approximately 300 milliwatts from battery 101. When transmitter 105 is in idle mode, it is not capable of transmitting RF signals but only draws 5 milliwatts from battery 101. As is well-known in the prior art, the particular protocol to which wireless terminal 100 operates determines how and when transmitter 105 toggles between transmit mode and idle mode.
Wireless terminal 100 can operate in either full-duplex or half-duplex mode. When wireless terminal 100 operates in full-duplex mode, both receiver 103 and transmitter 105 can operate simultaneously. Furthermore, receiver 103 can toggle between receive mode and idle mode independently of when transmitter 105 toggles between transmit mode and idle mode. In full-duplex mode the power drain on battery 101 can range from 10 milliwatts to 550 milliwatts.
In contrast, when wireless terminal 100 operates in half-duplex mode, either receiver 103 or transmitter 105 can operate, but not both simultaneously. Furthermore, neither receiver 103 nor transmitter 105 can toggle out of idle mode until the other has toggled into idle mode. In half-duplex mode the power drain on battery 101 can range from 10 milliwatts to 300 milliwatts.
Clearly, the duration with which wireless terminal 100 can operate is determined largely by the extent to which receiver 103 and transmitter 105 can remain in idle mode yet still provide an acceptable quality of service to the user of wireless terminal 100.
Another technique for lengthening the duration with which a wireless terminal can operate is to incorporate a longer-life battery into the wireless terminal. This is, however, not always practical because the market demand for a wireless terminal is based largely on the size and weight of the wireless terminal and a long-life battery is heavy and bulky and detracts from the marketability of the wireless terminal. There are, in fact, situations in which the form factor of the wireless terminal's radio simply will not accommodate a larger, longer-life battery. As is well-known in the art, the term "form factor" means the spatial dimensions that limit the size of a device.
Furthermore, there are situations in which the radio's form factor is so limiting that it is not possible to include both the radio and a battery that is large enough to merely power the radio into the allocated form factor. This is particularly true for the PCMCIA card or "PC" card form factor.
For example, when time-division receiver 103 and time-division transmitter 105 are designed to fit into a PCMCIA card form factor, time-division receiver 103 and time-division transmitter 105 may at times require up to 550 milliwatts, but the space remaining in the PCMCIA card for battery 101 may only leave enough room for a battery that can supply 200 milliwatts. Therefore, the need exists for a time-division radio whose transmitter and receiver can operate on a low-power battery.